Production of aryl tetracarboxylic acid anhydrides



Patented Jan. 13, 1953 PRODUCTION OF ARYL TETRACARBOXYLIC ACID ANHYDRIDES Robert J. Miller, Berkeley, Calif., assignor to California Research Corporation, San Francisco,

Califl, a corporation of Delaware No Drawing. Application October 8, 1947, Serial No. 778,740

9 Claims.

This invention relates to a process for the production of aryl tetracarboxylic acids and/or anhydrides from non-aromatic hydrocarbons, and particularly, to a process for the production of pyromellitic acid and/or anhydride from the non-aromatic compounds of'a petroleum hydrocarbon fraction.

Aryl tetracarboxylio acids, and particularly It has now beenfound that aryl tetracarboxylic acids and particularly the aryl tetracarboxylic acid anhydrides may be prepared in commercial yields from petroleum hydrocarbons 'or other non-aromatic hydrocarbons in accordance with the process of this invention. In its broader aspects, the subject process involves the aromatization of non-aromatic hydrocarbons of at least 6 carbon atoms per molecule, selective separation of an ortho-positioned tetramethyl benzene rich fraction therefrom, and oxidation of said orthopositioned tetramethylbenzenes to the tetracarboxylic 'anhydride. Thus; as distinguished from the prior methods of preparation, the subject process involves an abundant and inexpensive charge stock which is then processed through a series of relatively simple stages, some of which utilize existing petroleum refinery equipment, and all of which are a natural adjunct to conventional refinery practices, to yield the desired aryl tetracarboxylic anhydride or its corresponding acid.

The invention is best illustrated by reference to its preferred form in which a naphthenic petroleum distillate fraction containing an average of from 6 to 12 carbon atoms per molecule is subjected to aromatization by hydroforming over a vanadium or molybdenum oxide catalyst, fractionally distilling the resulting aromatics to separate a polymethylbenzene fraction containing at least 10 carbon atoms and boiling in the range of 380 394 R, chilling and crystallizing out a durene fraction therefrom. and' oxidizing said durene fraction to pyromellitic anhydride by a vapor phase oxidation over a vanadium oxidation catalyst.

In general, the feed stock to the process may comprise any non-aromatic hydrocarbon or mixtures thereof containing at least 6 carbon atoms per molecule. Although'there appears to be no upper limit as to the molecular weight of the hydrocarbon charge, it is of practical advantage to restrict their molecular weight to a maximum of about 14 carbon atoms per molecule. These hydrocarbons may be either acyclic compounds such as the paraflins, olefins and diolefins, or alicyclic compounds such as the alkylcyclohexanes, etc. The most desirable charge is one containing predominant amounts of alicyclic compounds which reduces the aromatizati-on stage to a dehydrogenation of the alicyclic nucleus and minimizes the formation of deleterious by-products.

The preferred charging stock is a petroleum hydrocarbon fraction within the molecular weight range of from 6 to 12 carbon atoms per molecule selected from anaphthenic crude such as the Coastal and Mid-Continent crudes. The hydrocarbons present in the preferred naphthenic petroleum fractions are, in general, alicyclic compounds containing a nucleus ring of 5 and 6 carbon atoms with some 7 carbon rings which possess alkyl 'substituents attached thereto of varying length and substituted in one or more positions.

The particular type of petroleum fraction found suited for the process of the invention is a naphthenic straight rungasoline fraction or distillate having a boiling point range of from 200 F. to 400 F. and preferably from 280 F. to

350 F. Naturally, theboiling range of the frac-j fied by the selection of a narrow boiling point fraction.

This non-aromatichydrocarbon charge stockis then converted to aromatic hydrocarbons by passing through the aromatization stage. The term aromatization as used in the subject specification and claims refers to the conventional conversion processes involving cyclization and/or 1 dehydrogenation reactions as required by the composition of the hydrocarbon feed stock. Thus, the aromatization of an acyclic hydrocarbon is fundamentally a cyclization and dehydrogenation reaction, whereas the aromatization of the alicyclic hydrocarbons is primarily a dehydrogenation reaction.

Furthermore, the separation The aromatization step as contemplated by this invention utilizes the conventional conversion processes involving the direct or incidental production of aromatics familiar to those skilled in the petroleum and chemical art. These aromatization processes include the so-called hydroiorming and catalytic reforming processes generally applied to naphthenic stocks; the "catalytic cyclization processes applicable to the paraflinic or acyclic type of stock; the "catalytic cracking processes such as the fluid and fixed bed catalytic cracking processes which produce aromatic hydrocarbons incidental to the primary cracking reactions; and the thermal aromatization reactions.

Of the various types of conversion processes which may be used as the aromatization stage, it is preferable to operate by the hydroforming process. In this process the naphthenic feed stock is introduced in the vapor phase to a conversion zone containing a hydroforming or dehydrogenation catalyst and contacted with controlled amounts of hydrogen. Although the reaction may be conducted with the conventional dehydrogenation catalysts, the best yields of arcmatics are obtained by the use of the metal oxide catalysts of the V and VI groups of the Periodic System, with particular preference given to the vanadium and molybdenum oxide catalysts.

In the aromatization stage, using a hydroforming process and a naphthenic charge, the conversion of the alicyclic hydrocarbons to aromatics occurs by a number of reactions. Primarily, the 6 carbon alicyclic nucleus is dehydrogenated to an aromatic nucleus while retaining the alkyl substituents attached to the residual nucleus. The alicyclic compounds containing and 7 carbon atom nuclei are apparently isomerized and dehydrogenated to additional aromatic com pounds, whereas the alkyl substituerrts attached to the various alicyclics may be altered in position and length by incidental cracking, alkyl transfer, and alkylation reactions.

By way of example, the following is an analysis of a typical hydroformed fraction boiling above 300 F. from commercial production:

Vapor Component or fraction temp; C Percent by volume Toluene 110.6 0.2. Ethyl Benzen 132. 2 0. 1. p-Xylene 138. 4 0.1. :11 Xylene 139.1 0. 3. o-Xylene 144. 4 1. 6. Cumene 152. i O. 03. n-lggtpyll benzene 139. 2 0. my ouene. 11.3 4. Infra. d t p-Ethyl toluene 162.1 3.1 we? Mesitylene 164. 7 4.1 we analysls' o-Ethyl toluene- 165. 2 3. 0. Pseudocumene- 169. Z 9. 8. Hemimellitene- 176. 2 5.0. m-Oymeue 175. 2 0.1. Dietlhyl benzenes, ethyl 179-194 3.8

xyenes. Durene, isodurene 194-200 1.0 Dlsmkmon analysls olylzlilblyll benzenes 203-543 6.3 8p aene'm Ultraviolet s t 2-methyl naphthalene 241.1 3.0} P l-methfloaphthalena. 244.8 3.0 meme analws Polyalkyl benzenes 240-255 0. 7 gimetihyllnaphttlgalenesifi-i. 10. [11

rime y an e y 11. naphthalenea Dsigillatiun analy Tetra and pentamethyl 285-295 5.7

naphthalenes. Bottoms Above 295 12.5

Following the aromatization stage the resulting mixture of aromatic compounds are fractionated to obtain a concentrated fraction of polymethyl benzenes of at least four alkyl groups per aromatic nucleus. This may be accomplished by solvent extraction, selective adsorption and desorption, or fractional distillation, depending upon the composition of the aromatic charge and the degree of selectivity required of the fractionation method. The composition of the aromatic stock will vary according to the particular process used in the aromatization stage and will contain varying amounts of lower alkyl benzenes and polynuclear aromatics, as well as the higher polyalkyl benzenes desired to be concentrated. In general, the boiling range of the desired polymethyl benzenes of at least 10 carbon atoms per molecule lie within the range of from 350 to about 410 F.

In the preferred form of the invention, the aromatics resulting from the hydroforming operation are subjected to fractional distillation and a tetramethyl benzene cut boiling in the range of 380-4l0 F. removed. The distillation may also be carried out in the presence of azeotroping agents such as Cellosolve or solvents such as phenol or aniline to sharpen the separation between the tetramethyl benzenes and the other aromatics of substantially the same molecular weight and boiling range. The lower molecular weight aromatics or the lower boiling constituents may be recycled to the hydrof-orming process and additional yields of tetramethyl benzenes obtained.

The polymethyl benzene fraction is next treated to selectively extract therefrom the methyl benzenes containing at least four methyl substitutes attached to the benzene nucleus in ortho-positioned pairs which are referred to in the present specification and claims as o-positioned tetramethyl benzene. This selective ex traction step is used to obtain a substantially pure charge of o-positioned tetramethyl benzenes for the oxidation reaction and minimize the loss of valuable aromatics which would be destroyed in the oxidation reaction. However, if the separation or fractional distillation is sharp enough to obtain a high durene concentration in the polymethyl benzene cut or if the economics warrant the loss of the accompanying aromatics, the selective extraction step may be avoided and the polymethyl benzene cut introduced directly to the oxidation stage of the process. The residual aromatics do not eiiect the oxidation reaction of the o-positioned tetramethyl benzenes, since they are largely decomposed to combustion products; but, at the same time, care must be exercised to avoid the efiect of this incidental combustion on the temperature control in the primary oxidation reaction.

In the preferred form of the invention, the tetramethyl benzene cut fractionally distilled from the hydroformed product is chilled to about -40 F. and the o-positioned tetramethyl benzenes such as durene are crystallized out and removed from the residual aromatics. The optimum chilling temperatures ar largely dependent upon the concentration of 'durene and the composition of the accompanying polyalkyl benzenes in the distillate cut. If the durene is sufliciently concentrated, it will crystallize out at room temperature or above. However, for maximum yields, the po'ly'methyl benzene out should be cooled just short of the point where the first eutectic with the accompanying compounds is formed. In practical application, the chilling temperatures should not be lower than -50 F.

The crude durene may be purified after the first crystallization by recrystallization either with or without the addition of solvents. Thus,

5. for the production of Substantially pure durene, the durene cake resulting from the filtration of the initially chilled product is remelted and cooled slowly to about 30 R, where it is filtered and washed to a durene of at least 90% purity.

The slow cooling allows the build-up of large durene crystals which are easier to filter, and minimizes the durene content of the filtrate. Suitable solvents for the recrystallization include the light aliphatic solvents such as petroleum ether, acetone, and ethyl alcohol, etc., which possess 'a relatively low solvent power for durene.

In the final stage of the subject process, the o-positioned tetramethyl benzenes resulting from the selective extraction or crystallization step are subjected to a catalytic vapor phase oxidation process to produce the desired aryl tetracarboxylic anhydrides or their acids. Although many of the conventional catalytic vapor phase oxidation processes may be used within the skill Y plished by scrubbing the effluent reactor gases with water and extracting.

The optimum conditions of oxidation resulting in maximum yields of acid or anhydride will vary with the composition of the charge, nature of c the catalyst, and the type of reaction system employed. The catalysts used in the preferred process are of the type normally referred to as a vanadium oxidation catalyst. These catalysts consist primarly of the compounds of vanadium which, at elevated temperatures, readily interchange the valence states of vanadium, such as the oxides of vanadium and other compounds of vanadium such as tin vanadate. In practice, these vanadium catalysts are supported on an inert carrier of low surface to volume ratio, such as silicon carbide or alumina, and are pelleted or ground to a particle size in conformity with the 1 type of reactor or conversion system used.

Operable temperature range for effecting the oxidation is limited at the lower extremity by the activity of the particular catalyst. For example, vanadium oxide, which is the preferred catalyst, possesses a minimum operation temperature of about 700 F., since, below this temperature, the oxide cannot be reoxidized from the lower to the higher valence state by air or oxygen alone. The minimum operating temperatures at which vanadium may be regenerated to its higher valence states will vary with the composition of the catalyst. Thus, tin vanadate is most eifective in the region of 550 F. The maximum oxidation temperature will depend upon the stability of the desired carboxylic anhydrides. For practical purposes, 1200 F. is used as the maximum temperature of operation. Above about 1200 F. the

pyromellitic anhydride becomesunstable and resultant yields of product. are decreased. 3 Since the oxidation reaction is highly exo thermic, particular care must be taken to insure The oxidation products will adequate temperature control in 1 the catalystchamber. From the standpoint of temperature control, it is advantageous to use the so-called- Fluid Catalyst type of operation, which utilizesi low gas velocities and a powdered catalyst. The mechanics of this type of reaction system is dis-. closed in Petroleum Refiner, 1947 Process Hand Book, Section I (April 1947), pages 150-153.

When using a fixed bed catalyst operation, it has been found desirable to conduct the reaction in a reactor resembling a heat exchanger in" which the tube bundle is packed with catalyst, and a suitable coolant, such as molten salt or boiling mercury, is circulated through the shell.

small, say, inch inside diameter, satisfactory" temperature control has been obtained; There is still sufiicient heat of reaction to create a local hot spot in each reactor tube which must be con-' trolled by variables other than bath temperature.

It is preferred to maintain this hot spot temperature between 800 and 1100"F. A further aid in maintaining temperature control in this type of reactor is the use of relatively coarse catalyst particles. Mesh sizes from 4 -10 have proven satisfactory.

In general, the vapor phase oxidation reaction is conducted with an excess of oxygen over that theoretically required for conversion to the anhydrides. When using a fixed bed reactor, and air as the oxygen-containing gas, the oxidation may be carried out with an air-to-hydrocarbon ratio in the range of to 1000 by weight, but preferably the ratio is kept between and 200 to ole-- tain the best yields commensurate with high throughput.

The preferred process of the invention is shecifically illustrated by reference to the following example in which a hydroforming process is used as the aromatization stage for the production of durene and the oxidation of dureneto pyromellitic anhydride is carried out in a fixed bed reactor consisting of a single three-foot, stainless steel reaction tube of inch inside diameter, jacketed with a mercury bath whose boiling temperature is controlled with nitrogen pressure, and packed with about to cc. of vanadium pentoxide catalyst.

Example 1 A to 350 F. boiling range distillate from California Waxy Crude was subjected to catalytic hydroforming at 900 to 1000 F. over a coprecipitated aluminum moiybdate catalyst withadded hydrogen. The liquid product was distilled and the fraction in a boiling range of from 380 to 410 F. was removed. This distillate wasthen cooled to 40 F. and the crystalline phase filtered and purified by recrystallization at 150 F.

to produce pure durene. Additional durene was obtained by chilling the filtrate from the re-' crystallization to 40 F. and reworking. The

yield of durene from the 380-410" F. cut was 20.5%. The durene was then heated to 280 F mixed with a preheatedstream of air and introduced into a fixed bed catalytic oxidation reactor.

The resultant homogeneous gaseous mixture was condensers in which the py-romellitic anhydride;

By keeping the diameter of the tubes sufficiently 7'. and other products of low volatility were collectedas solids. An analysis of the condenser products after one hour of; operation resulted in 22%. grains of crude. pyromellitic anhydride from 31.3, cc. (25.3 grams) of durene, or 89.0 weight per cent.- conversion.

In orderto increase, the yield of durene from the hydroforming stage, the lower boiling constituents of. the hydroformate may be recycled back to the, aromatization unit. Thus, a 300 to 360 F; boiling range fraction from the first pass was: recycled and treated under the same conditions of hydroforming as set forth in the foregoing example, On fractional distillation, a 362+4ill F. out was removed and worked up to obtain a yield of durene, rom this cut. Similarly, a. 270 -300 F. boiling range fr ction was recycled and the product, fractionally dis tilled to a 370-41 0- out which worked up to a durene yield of 8%.

Example.- 2

A 300 to 360 F. boiling range distillate from California Waxy Crude was S02 treated to reduce the aromatic hydrocarbon content to 2%. The rafiinate was then catalytically reformed over a coprecipitated aluminum molybdate catalyst with added hydrogen at a space rate of 1 liquid volume per volume of catalyst per hour with an inlet temperature of 985 F. and an average catalyst temperature of 927- F. The liquid product was then fractionally distilled and a distillate cut boiling range from 383 to 411 F, was removed. This distillate cut was then cooled to F. and the resulting crystals separated and purified by recrystallization at 150 F. to substantially pure durene. Additional durene was obtained by chilling the filtrate to -40 F. and reworking. The yield of pure durene from the 383 to 411 F. fraction was 16%. The purified durene was then heated to 280 F., mixed with preheated air and charged to the fixed bed catalytic oxidation reactor. The air rate in the oxidation charge was maintained at 53 mols per hour While the liquid durene was pumped at the rate of 20.7 cc. per hour. The homogeneous gaseous charge was passed over a catalyst composed of 4 to 6 mesh Alundum coated with vanadium pentoxide maintained at a reactor temperature of 1050 F. The oxidation products were then passed through aircooled condensers where the pyromellitic anhydride was collected as a solid. An analysis of the reaction products indicated a 102 weight per cent conversion to pyromellitic anhydride of 94 purity, based on neutral equivalent.

Additional runs were made in which the conditions of the oxidation reaction were modified to illustrate the effect of various air-hydrocarbon ratios. Thus, a gaseous mixture of durene and air was. char ed to the reactor packed with 100 cc. of 4, to 0 mesh vanadium pentoxide on Alundum catalyst at a durene rate of 15 cc. per hour and an air rate of 63 mole per hour with a bath temperature in the reactor of 890 F. The oxidation conversion was a 102.5 weight per cent yield of 97.3% pure pyromellitic anhydride. This run was duplicated, except that the air rate was decreased to 44 mols per hour and the durene rate increased to 30.0 co. per hour. The yield obtained was 80.0 weight per cent of 96.2% pure pyromellitic anhydride.

Example 3 A 300 to 350 F. boiling range distillate from a California Midway Crude was subjected to catalytic hyd o ormin at 900-1000 E- q a. 9.-

precip tated' alu inum mo bdate ca a s ithv added hydrogen, The, liquid product was distilled and a fraction in the boiling range of from 370-392 F. was removed. This cut was then chilled to +40 F. and a durene rich crystalline phase separated by filtration. The durene cake was then melted and recrystallized at 150 F. to produce substantially pure durene. Additional durene was obtained by chilling the filtrate from the recrystallization to 740 E. and reworking. e d. of: d r e rom. e S o 9 bo li range fraction was 22% by weight. The durene was then heated to 280 F. and metered through a pu pi sy t a the rate. f 2. ocper ho r to a vaporizer where the durene was contacted h. a t ed. stre m of preh a d ir lo in a a rat of o e. er h r The homogeneous gaseous mixture was then introuc d to a. fixe bed cat yti v oxida i n o The r actor as. pac ed ith 5-11 m s an i m oxide on Carborundum and was maintained at a reactor temperature of 350 and a hot spot temperature of 1050" F. The eiiluent oxidation products were then passed through a pair of aircooled condensers where the pyrornellitic anhydride was collected as a solid. The results obtained indicate a 93.7 weight per cent yield of 93.5% pure pyromellitic anhydride based on neutral equivalent.

The following example is presented as a specific illustration of the preferred method of producing substantially pure durene.

Egtample 4 The first pass bottoms (18 API gravity) from a commercial catalytic hydroforming plant were fractionallydistilled to obtain a distillate fraction boiling in the range of 350-410 R, which analyzed to a durene content of 17%. This distillate cut was then cooled rapidly by adding Dry Ice to the stock, and the resulting slurry was filtered at 4=0 F. The filtration produced a filter cake containing 43 durene. The durene cake was then melted with steam coils and allowed to crystallize slowly from 120 F. to 70 ER, forming fiat hexagonal crystals of approximately 1-3 millimeters across. The crystalline slurry was then filtered and washed with a 250 F. boiling point naphtha solvent at 30 F. to produce a product of durene and 10% solvent and water. This product was then fractionally distilled to remove the solvent and obtain a solid, crystalline durene of 97% purity.

Obviously, many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and therefore only such limita;.

oarboxylic acid anhydrides which comprises pass- I ing a petroleum distillate fraction containing at least 6 carbon atoms per molecule through'a catalytic vapor phase conversion zone maintained under aromatization conditions, fractionally distilling the resulting aromatic hydrocarbons to obtain a fraction within the boiling range of from 350 to 410 F., selectively separating from said fractional distillate a durene rich fraction, subjecting said durene rich fraction to a catalytic vapor phase oxidation and recovering the resulting pyromellitic acid anhydride.

3. A process for the production of pyromellitic acid anhydride which comprises introducing a non-aromatic hydrocarbon charge containing at least 6 carbon atoms per molecule into a vapor phase catalytic conversion zone maintained under aromatization conditions, fractionally distilling the resulting aromatic hydrocarbons to obtain a polymethyl benzene fraction boiling in the range of 380-410- F., selectively crystallizing from said distillate cut a durene rich fraction and catalytically oxidizing said durene in the vapor phase to pyromellitic acid anhydride.

4. A process for the production of pyromellitic acid anhydride which comprises subjecting a naphthenic petroleum distillate containing at least 6 carbon atoms per molecule to a catalytic hydroforming process, fractionally distilling the resulting aromatic hydrocarbons to obtain a distillate out boiling in the range of about 380-410 F., oxidizing said distillate cut in the vapor phase over a vanadium oxidation catalyst, and recovering pyromellitic acid anhydride.

5. A process for the production of pyromellitic acid anhydride which comprises fractionally distilling the aromatic products resulting from a petroleum hydroforming process to obtain a distillate cut boiling in the range of 350-410 F., chilling said distillate cut, separating therefrom a crystalline durene concentrate and oxidizing said durene concentrate in the vapor phase over a vanadium oxidation catalyst to the corresponding pyromellitic acid anhydride.

6. A process for the production of durene which comprises subjecting a non-aromatic hydrocarbon charge stock containing at least 6 carbon atoms per molecule to a catalytic conversion zone maintained under aromatization conditions, fractionally distilling the resulting aromatic hydrocarbons and removing a distillate fraction boiling in the range of 350-410 F., chilling to a temperature above the first eutectic point of said distillate cut and selectively extracting therefrom durene in a crystalline phase.

7. A process for the production of durene which comprises subjecting a naphthenic petroleum distillate containing at least 6 carbon atoms per molecule to a catalytic hydroforming process,

fractionally distilling the resulting aromatic hydrocarbons to obtain a distillate cut boiling in the range of 350-410 F., chilling to a temperature above the first eutectic point of said distillate cut and selectively extracting therefrom durene in the crystalline phase.

8. A process for the production of pyromellitic acid anhydride which comprises fractionally distilling the aromatic hydrocarbons resulting from a vapor phase catalytic conversion process maintained under aromatization conditions to remove a distillate cut boiling in the range of 350-410 F., selectively extracting from said distillate cut a durene concentrate and oxidizing said durene concentrate in the vapor phase over a vanadium oxidation catalyst to the corresponding pyromellitic acid anhydride.

9. A process for the production of benzene tetracarboxylic acid anhydrides which comprises subjecting a petroleum distillate fraction containing at least 6 carbon atoms per molecule to an aromatization process, fractionally distilling the resulting aromatic hydrocarbons to obtain a fraction within the boiling range of 350-410 F., selectively extracting from said fractional distillate an ortho-positioned tetramethyl benzene fraction and subjecting said ortho-positioned tetramethyl benzene hydrocarbons to a vapor phase oxidation over a vanadium oxidation catalyst.

ROBERT J. MILLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 22,930 Carpenter Oct. 21, 1947 1,834,679 Canon et a1 Dec. 1, 1931 2,320,147 Layng et a1 May 25, 1943 2,338,573 Creelman Jan. 4, 1944 2,404,902 Claussen et al July 30, 1946 2,425,559 Passino et al Aug. 12, 1947 2,474,002 Levine et al June 21, 1949 OTHER REFERENCES Mair et al. Jour. Rec. Nat. Bur. Stand, vol 27, 343-349 (1941).

Richters Organic Chemistry, A. J. Mee, vol. III, 3rd Ed. 1946, page 396.

Sachanen The Chemical Constituents of Petroleum, Reinhold Pub. Corp. N. Y. (1945) 221, 222, 259, 260. 

1. A PROCESS FOR PRODUCTION OF ARYL TETRACARBOXYLIC ACID ANHYDRIDES WHICH COMPRISES SUBJECTING A NON-AROMATIC HYDROCARBON CONTAINING AT LEAST 6 CARBON ATOMS PER MOLECULE TO AN AROMATIZATION PROCESS, SEPARATING FROM THE RESULTING AROMATIC HYDROCARBONS A DURENE RICH FRACTION AND CATALYTICALLY OXIDIZING SAID DURENE RICH FRACTION IN THE VAPOR PHASE TO THE ARYL TETRACARBOXYLIC ACID ANHYDRIDE. 